The geometric dimensions of antennas are closely related to the wavelength of the waves which are intended to be emitted or received via the antennas. Frequently the space available for the antennas is restricted (in the case of portable radio appliances, for example). Consequently, the antennas should be designed to be as compact as possible, without restricting the bandwidth or the efficiency of the antennas.
A number of proposals have already been made in the past for folded monopoles to be used as compact antennas. These monopole antennas are arranged in the form of a hairpin above an electrically conductive base plane, with one end being conductively connected (grounded) to the base plane, while the other end is used as a feed point independently of the base plane. See, for example, U.S. Pat. Nos. 3,295,137 and 3,508,271.
The characteristic properties of unfilled and filled monopoles folded two or more times have also been investigated in an article by B. J. Lamberty, “A Class of Low Gain Broadband Antennas”, 1958 IRE Wescon Convention Report, pp. 251-259 (August 1958).
More recently, folded monopoles have been used to allow a radio link for portable computers (see, for example, U.S. Pat. No. 6,054,955) or to provide wire-free price labeling systems via LANs (see, for example, U.S. Pat. No. 5,668,560).
Vehicle antennas for use in the railroad field are subject to unique requirements. Local antennas such as these should be designed in individual cases for a frequency range from 870 MHz to 2170 MHz with a VSWR of <2, thus being suitable for the GMS 900, GSM 1800 and UMTS ranges. They should also be compact and mechanically robust in order to allow a reliable radio link despite the vibration, bugs and other environmental influences. Furthermore, owing to possible contact with the overhead wire on electrified sections of track, local antennas must, in accordance with the relevant test instructions, withstand in particular voltages of 16.6 kV and currents of 40 kA, in which case a voltage of no more than 60 V may occur at the RF connection, in order that personnel in the locomotive are not endangered by the downward feed cable. Examples of local antennas such as these are the K70 20 21 type antennas for the frequency range from 410 to 470 MHz, and the 742 325 type for the frequency range from 870 to 2170 MHz from the Kathrein Company. A broadband vehicle antenna which is resistant to heavy currents is also described in DE-A1-199 24 349.